Circuit breakers are special switches or protective or/and switching devices which are designed in such a manner that they can switch load currents, high overload and short-circuit currents on and off and can safely disconnect them under many fault conditions, for example a ground fault, phase opposition, an overload and a short circuit, in an electrical power supply system, for example a low-voltage network. Such switching devices are used in electrical installations as feed switches, distribution switches, coupling switches and outgoer switches. Such switches are also used when switching and protecting motors, capacitors, generators, transformers, busbars and cables.
Circuit breakers can be used, in particular, in low-voltage networks, that is to say networks with voltages or rated voltages of up to 1000 volts AC or 1500 volts DC. These circuit breakers have sensor units, such as current sensors, which measure the electrical current flowing through switches.
Circuit breakers are used for current intensities or a rated current range of 16 to 6300 amperes, in particular in the low-voltage range. They are used as a molded case circuit breaker, for example, in particular in the range of 16 to 1600 amperes, more specifically in the range of 63 to 1600 amperes, and are used as an air circuit breaker, for example, in the range of 630 to 6300 amperes.
Circuit breakers usually have a tripping unit for interrupting the electrical circuit, which tripping unit is implemented, for example, using contacts which open and close the electrical circuit. A circuit breaker also has a control unit which may be implemented in a central or decentralized manner. If current limit values or/and current/period limit values, which are defined by a characteristic curve for example, are exceeded, the circuit breaker interrupts the electrical circuit.
Circuit breakers are designed for different currents. A first current characteristic value of the circuit breaker is the rated current In. This is generally the current with which the circuit breaker can be permanently loaded. This rated current depends on the device. One or more current limit values which are less than, equal to or greater than the rated current In are generally defined or adjusted.
A first adjustable current limit value Isd of the circuit breaker, which must be present for a first period t1 for an interruption, is the current limit value for the so-called interruption with a short time delay or current-dependent interruption with a short time delay. That is to say, this first current limit value Isd defines the level of the current from which an interruption with a short time delay begins. In this case, the current must be present for the first period t1 before an interruption is affected.
An adjustable second period tsd of the circuit breaker, during which a second current limit value Iref must be present for this second period, defines the minimum period for the interruption with a short time delay or current-dependent interruption with a short time delay before which tripping/interruption is effected for the maximum second current limit value Iref which can have a short time delay. If the current increases further, the electrical circuit is generally interrupted without a delay, that is to say an interruption is effected with the minimum or adjusted short or shortest tripping time of the circuit breaker.
The range between the first current limit value Isd during the first period t1, that is to say the start of the method of operation or characteristic curve with a short time delay, and the second current limit value Iref during the adjustable second period tsd, that is to say the end of the method of operation with a short time delay, is defined by a characteristic curve in which the period decreases with increasing current in order to effect an interruption.
Circuit breakers are used to distribute electrical energy. If a fault occurs in the energy distribution installation, the circuit breaker arranged immediately upstream of the fault is supposed to detect the fault or/and short-circuit current, trip the circuit and disconnect the current. A plurality of parameters, current set values, delays and characteristic curve forms are required for this so-called selective tripping.
The devices can be selectively tripped, for example, using current or time staggering. In this case, the characteristic curves of the devices must not overlap. FIG. 1a illustrates an example of a logarithmically represented current/period limit value characteristic curve. FIG. 1a shows a graph, where the current I is plotted on the X axis and the time t is plotted on the Y axis. FIG. 1b shows a series circuit comprising a first circuit breaker O1 which is arranged on the current-source side or feed side and is followed by a first busbar SCH1 for distributing the electrical energy. A second circuit breaker O2 is arranged on this first busbar SCH1 and is followed by a second busbar SCH2 for distributing the electrical energy further. A third circuit breaker O3 is arranged on this second busbar and one or more electrical loads are arranged at the third circuit breaker, for example. Figure la shows the characteristic curves of the circuit breakers O1, O2, O3, where these characteristic curves do not overlap.
In addition to the staggering of the characteristic curves of circuit breakers, as illustrated in FIG. 1, FIG. 2 shows an illustration according to FIG. 1 with the difference that a fuse F1, such as a low-voltage high-rupture-capacity fuse, is connected downstream of the first circuit breaker O1. Since the characteristic curves of circuit breakers and fuses differ, an accordingly large distance between the characteristic curves should be selected in order to achieve selectivity since the characteristic curve form of fuses virtually cannot be adapted and the characteristic curve form of circuit breakers can be adapted only to a limited extent.
The current/period limit value characteristic curve of the circuit breaker O1 according to FIG. 2a has a reference point RP at which the current/period limit value characteristic curve changes to a fixed delay. The position of the reference point RP depends on the device.
As a result, a selective or non-selective behavior can result on the basis of the circuit breakers used or the upstream or downstream fuses.